Relaying is being considered for Long Term Evolution (LTE) Rel-10 as a tool to improve the coverage of high data rates, group mobility, temporary network deployment, the cell-edge throughput and/or to provide coverage in new areas. At least type 1 relay nodes are part of LTE-Advanced, and a type 1 relay is an inband relaying node (RN).
To avoid self interference at relay, and at the same time maintain backward compatibility to Release 8 (Rel-8) user equipment units (UEs), Multicast Broadcast Single Frequency Network (MBSFN) subframes configured for the relay-to-UE link are used to create transmission gaps for eNB-to-relay transmissions, as illustrated in FIG. 1. In the illustrated example relay-to-UE communication using normal subframes and eNodeB-to-relay communication using MBSFN subframes
If time-aligned subframes are used for evolved NodeB (eNB) and relay transmissions, the relay can not receive the Rel-8 control signalling from its eNB during the first 1 or 2 Orthogonal Frequency-Division Multiplexing (OFDM) symbols as the relay has to transmit the Physical Downlink Control Channel (PDCCH) to user equipment units in the relay cell at the same time. To solve this problem, in Third Generation Partnership Project (3GPP) it has been discussed the use of a new control channel structure, i.e. the Relay-Physical Downlink Control Channel (R-PDCCH), for the backhaul link between the donor cell and the relay node. The R-PDCCH is transmitted outside the control region in the donor cell using a semi-statically configured set of Resource Blocks (RBs) as illustrated in FIG. 2.
A time-shifting approach was also proposed to enable the relay to receive control signalling from its donor eNB. The basic idea of time shifting is to use non-time-aligned subframes for eNB and relay transmissions so that the Rel-8 control channel structures can be reused as much as possible. By doing this, relay can receive control signalling in relay-to-UE transmission gap. Time shifting is illustrated for Time Division Duplex (TDD) in FIG. 3.
Defining a dedicated R-PDCCH, and possibly also related channels such as Relay Physical Hybrid Automatic Repeat reQuest (HARQ) Indicator Channel (R-PHICH) and Relay Physical Control Format Indicator Channel (R-PCFICH) requires a substantial design effort, revisiting many of the lengthy discussions and decisions taken during the Rel-8 design. The control channel design might also impact other areas such as reference signals.
Time shifting could be a solution to reuse LTE Rel-8 design and thus to simplify the overall standardization and implementation effort. However there are some disadvantages for such a scheme
As the frames of eNB and relay are not time-aligned, some LTE Rel-10 features e.g. CoMP and MBSFN schemes between eNB and relay can not be used.
As the last few OFDM symbols in a subframe can not be received (some form of overhead), which can be regarded as overhead, the transmission efficiency is reduced.
In TDD systems, RN undergoes more severe Tx-Rx switching loss than in FDD systems. As increased downlink-uplink switch time such as i.e. few OFDM symbols is needed, the downlink-uplink switch time will be reduced for a given special subframe configuration, which leads to reduced coverage. Larger guard periods may be utilized, which however leads to additional overhead.